15 - Renal Physiology

7/30/2019 15 - Renal Physiology

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Lecture: Renal Physiology

I. Overview of Nephron Structure and Function

A. General Nephron Structure

1. glomerulus - site of filtration from arterial blood

2. proximal convolute tubule- first tube off glomer.

3. Loop of Henle - U-turn connecting tubules

4. distal convoluted tubule - to the Collecting Tubule

5. collecting tubule - urine from many nephron6. peritubular capillaries - "around" the "tubes"

B. General Nephron Function

1. glomerular filtration

2. tubular reabsorption

3. tubular secretion

C. Fluid Processing in the Kidneys

180 liters of blood fluid processes each day

1.5 liters of urine produced each day

II. Glomerular Filtration

A. Filtration Membrane

1. hydrostatic pressure - forces 1/5 of blood fluid through capillary' walls into glomerular

capsule

2. filtration membrane - has three parts

a. fenestrated capillary endothelium (prevents passage of blood cells)

b. basal membrane (allows most solutes but larger proteins)c. visceral membrane of glomerular capsule

3. solutes that can pass into glomerular capsule

< 3 nm easily pass (water, sugar, amino acids, nitrogenous waste molecules)

> 9 nm larger proteins cannot pass through

B. Net Filtration Pressure

NFP = force OUT of blood - force to remain IN blood

NFP = glomerular - (glomerular + capsular )

hydrostatic osmotic hydrostatic

pressure pressure pressure

NFP = 55 mm Hg - ( 30 mm Hg + l5mmHg)NFP = 55 mm Hg - (45 mm Hg)

NFP = net filtration pressure = 10 mm Hg

[This is the NET forces pushing fluid/solutes OUT of blood]

1. glomerular filtration rate = milliliters of blood fluid filtered by glomerulus each minute

Factors effecting the GFR:


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a. total filtration surface area

b. membrane permeability to fluid/solutes

c. Net Filtration Pressure

2. Normal GFR = 125 ml/min (7.5 L/hr, 180 L/day)3. NFP - primary factor controlling GFR

a. bleeding - NFP drops, lowers the pressureb. dehydration - NFP drops, lowers the pressure

D. Intrinsic Controls: Regulation of Glomerular Filtration

1. renal autoregulation - rate of FILTRATE production must be coordinated with

reabsorption rate2. myogenic mechanism - circular muscle around the glomerular arterioles reacts to

pressure changes

a. increased blood pressure -> vasoconstriction

b. decreased blood pressure -> vasodilation

3. tubuloglomerular feedback mechanism - macula densa cells (of juxtaglomerular

apparatus) sense the solute concentration of the FILTRATE

a. low concentration > vasodilation

b. high concentration -> vasoconstriction

4. renin-angiotensin mechanism

renin (released by juxtoglomerular cells) -> angiotensinogen -> angiotensin I -> angiotensin II ->

global vasoconstrictor (rise in blood pressure) release of aldosterone (resorption of more Na+)

Factors causing release of Renin:

a. reduced stretch of juxtaglomerular cells

b. stimulation by macula densa cells (as above)

c. stimulation of juxtaglomerular cells by sympathetics

E. Extrinsic Controls: Sympathetic Innervation

1 sympathetics - cause increased release of renin

2 epinephrine - causes increased vasoconstriction

III. Tubular Reabsorption: Reabsorbing the Glomerular Filtrate

A. Overview of Reabsorption

1. filtrate - all fluid and its solutes pushed into the capsule

2. urine - filtrate minus reabsorbed substances + secreted substances3. route of reabsorption (transepithelial process)

luminal surface of tubule cells >>

basolateral membrane of tubule cells >>

interstitial fluid between tubule cells and capillaries >>

endothelium of the peritubular capillary

4. most sugars and amino acids are reabsorbed


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5. water and ion reabsorption depends on hormonal control

B. Active Tubular Reabsorption

1. glucose, amino acids, lactate, vitamins, ions

a. move across luminal surface by diffusion

b. actively transported across basolateral membranei. cotransported with Na+

c. diffuse into capillary by diffusion

2. transport maximum (Tm)when "carrier proteins" for specific solute becomes saturatedand cannot carry the substance across the membrane

a. diabetes mellitus - lower Tm (glucose lost)

C. Passive Tubular Resorption

1. Na+ driven into interstitial space actively (above)

2. HCO3- and Cl- follow Na+ into the space

3. obligatory water resorption - water follows ions into the interstitial space between tubule& capillary

4. solvent drags - solutes will begin to move into tubule from filtrate, following water

(especially some urea and lipid-soluble molecules)

D. Nonreabsorbed Substances

1. urea, creatinine, uric acid - most is not reabsorbed because of the following reasons

a. no carrier molecules for active transport

b. not lipid-soluble

c. too large (as with most proteins)

E. Absorption in Different Regions of Renal Tubule

1. proximal tubule - closest to the glomerular capsule

a. almost all glucose & amino acids

b. 75-80% of water and Na+

c. most active transport of ions

2. Loop of Henle - connects proximal & distal tubules

Regulates Total water retained or lost:

a. descending limb - water can return to blood vessels

b. ascending limb water impermeable but releases ions to the interstitial space

increasing osmotic pressure so that water can be reabsorbed from other parts ofthe renal tubule

3. distal tubule & collecting duct - final passageway

a. antidiuretic hormone (ADH) - causes increased permeability to Na+ and water,

allow resorption

b. aldosterone - stimulated by renin-angiotensin, enhances Na+ resorption (water

follows). Triggered byi. lower blood pressure


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ii. low Na+ concentration (hyponatremia)

c. atrial natriuretic factor (ANF) - reduces Na+ permeability, less water (in response to high

B.P.)

IV. Tubular Secretion

A. Movement from Capillaries to Tubular Cells

1. K+, creatinine, ammonia, organic acids, drugs

2. Primary functions of tubular secretion:

a. moving drugs into the urine

b. moving more urea & uric acid into urine

c. removing excess K+ from blood

d. regulating pH (H+ ion removal)

V. Regulation of Urine Concentration & Volume

A. Osmolarity - Number of Solute particles in 1 Liter water

1. independent of size of solute (Na +, glucose)

2. 1 osmol = 6.02 X l023 particle in I Liter

3. milliosmol (mosm) = 0.001 osmol

4. normal body fluids = 300 mosm

B. Countercurrent Multiplier Mechanism for Maintenance of Blood/Urine Osmolarity

1. Water moves out along Descending Limb of the Loop of Henle, creating 1200 mosm

urine at the base2. Na+Cl- moves out along the Ascending Limb of the Loop of Henle, creating 100 mosm

urine at distal end. This salt helps pull more water out of the Descending Limb in positivefeedback mechanism.

3. In times of dehydration, Collecting Tubules leak urea to interstitial space, further

increasing water retention by increasing osmolarity.

4. Vasa recta (capillaries around Loop of Henle) have no Net Effect on water/salt balance

C. Formation of Dilute Urine

1. When water removal is needed, no ADH is released, so that the Distal and CollectingTubules will not actively transport Na+ out; no water moves out

2. Urine may be as low as 50 mosm

D. Formation of Concentrated Urine (Water Conservation)

1. antidiuretic hormone (ADH) - stimulates resorption of water in the Distal and Collecting

Tubules

E. Diuretics (Stimulate Water Loss)


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1. alcohol inhibits action of ADH

2. caffeine - causes renal vasodilation; increases GFR

3. Na+ resorption blockers - block Na+ movement

VI. Renal Clearance

A. Renal Clearance (RC) - the rate at which the kidney can remove a substance from the blood

RC = U/P X V

U/P = concentration of substance in urine (mg/ml) concentration of substance in plasma (mg/ml)V = rate of the formation of urine (ml/minute)

(normal = 1 ml/minute)

B. Glomerular Filtration Rate = 125 ml/minute; (determined by challenge with "Inulin")

1. RC < 125 - reabsorption is occurring

2. RC > 125 - tubule cells secrete into the urine

VII. Characteristics and Composition of Urine

A. Physical Characteristics

1. color - clear to yellowish; influenced by diet, drugs, and health state

2. odor - slightly aromatic; influenced by diet, drugs, and health state3. pH (H+ conc.) - usually about 6; changes in diet can effect the pH

4. specific gravity - compared density to distilled water; urine slightly heavier (with solute

s)

B. Chemical Composition

1. 95% water

2. 5% solutes - urea (breakdown of amino acids); uric acid; creatinine

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15 - Renal Physiology